1. Field of the Invention
The present invention concerns a method to detect incorrect magnetic resonance (MR) data in k-space representing MR signals acquired from an examination subject. In particular, the invention concerns a suitable replacement of such incorrect MR data.
2. Description of the Prior Art
Magnetic resonance tomography (MRT) is an imaging method that is used in many fields of medicine for examination and diagnosis. MRT is based on the physical effect of nuclear magnetic resonance. To acquire magnetic resonance signals, a static (B0) basic magnetic field is generated in the examination region at which the nuclear spins or, respectively, magnetic moments of the atoms in the examination subject align. The nuclear spin can be deflected from the aligned position via radiation of radio-frequency electromagnetic radiation (RF pulses). The relaxation from this excited position generates a decay signal that can be detected inductively by means of an acquisition coil.
By the application of a slice-selection gradient upon radiation of the RF pulses, nuclear spins are excited only in a slice of the examination subject. An additional spatial coding can ensue during readout by the application of a phase coding gradient and a frequency coding gradient. It is also possible to implement phase coding in two or three spatial directions for 3D imaging, two spatial directions are phase-coded, and frequency coding ensues in the third spatial direction. The coding by means of the magnetic field gradients spans k-space; a frequency-coded MR signal essentially corresponds to a line in k-space. K-space can be filled line by line via switching (activating/deactivating) different phase coding gradients. Image data can be reconstructed from such a digitized raw data set by Fourier transformation. A point in k-space corresponds to a specific spatial frequency in image space based on this relationship.
In the acquisition of magnetic resonance signals by means of the acquisition coils, single or multiple data points in k-space can be affected by errors, for example if they are overlaid with strong noise or interference signals/interference spikes. These errors, known as “spike noise”, lead to stripe artifacts in reconstructed images. The quality of reconstructed MR images is thus significantly negatively affected by spike noise.
To circumvent this problem, in some conventional magnetic resonance systems a particular antenna is used that is mounted at a rear part of the magnetic resonance system. If such an interference spike is detected by means of the antenna, the corresponding k-space point is set to zero. Although this simple method enables a certain and automatic discovery of the interference spikes, additional apparatuses for this purpose (for example the antenna, wiring, receiver etc.) are required, and the correction by setting the corresponding value to zero is very imprecise and leads to a reduction of the image quality.
From U.S. Patent Application Publication No. 2002/0084782 A1a method is known with which interference spikes are detected by measurement of the amplitude of signals having frequencies near the frequencies used in the MR experiment (nearband signals). If the signals exceed a specific threshold, the presence of interference spikes is established. This method also requires additional devices. If the same antenna as is used for the MR measurement (data acquisition) is used for the interference spike detection, the signal for interference spike detection must be diverted (tapped) before the further processing of the MR signal.
Such threshold methods are disadvantageous, in particular in the central k-space region where the variation of the MR signals is large. Furthermore, the setting of an incorrect data point to zero or the interpolation of such a data point from adjacent data points leads to false results, and thus to an unsatisfactory reproduction of the incorrect data point. This causes a reduced image quality. Methods that use filtering in image space to remove the resulting artifacts are likewise disadvantageous. These techniques normally make assumptions about the content of the image data that are likewise subject to errors.
It is accordingly desirable to reliably detect interference spikes in acquired magnetic resonance signals without additional devices and to replace them with suitable values. In particular, the replacement should negatively affect the image quality of reconstructed image data as little as possible.